In a typical wireless or radio communications network, wireless devices, also known as mobile stations, terminals, and/or User Equipment, UEs, communicate via a Radio-Access Network, RAN, with one or more core networks. The RAN covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g. a radio base station, RBS, or network node, which in some networks may also be called, for example, a “NodeB”, “eNodeB” or “eNB”. A cell is a geographical area where radio coverage is provided by the radio base station at a base station site or an antenna site in case the antenna and the radio base station are not collocated. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. Another identity identifying the cell uniquely in the whole mobile network is also broadcasted in the cell. One radio base station may have one or more cells. The base stations communicate over the air interface operating on radio frequencies with the user equipment within range of the base stations.
A Universal Mobile Telecommunications System, UMTS, is a third generation mobile communication system, which evolved from the second generation, 2G, Global System for Mobile Communications, GSM. The UMTS terrestrial radio-access network, UTRAN, is essentially a RAN using wideband code-division multiple access, WCDMA, and/or High-Speed Packet Access, HSPA, to communicate with user equipment. In a forum known as the Third Generation Partnership Project, 3GPP, telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. In some versions of the RAN, as e.g. in UMTS, several base stations may be connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller, RNC, or a base station controller, BSC, which supervises and coordinates various activities of the plural base stations connected thereto. The RNCs are typically connected to one or more core networks.
Specifications for the Evolved Packet System, EPS, have been completed within the 3rd Generation Partnership Project, 3GPP, and this work continues in the coming 3GPP releases. The EPS comprises the Evolved Universal Terrestrial Radio-Access Network, E-UTRAN, also known as the Long-Term Evolution, LTE, radio access, and the Evolved Packet Core, EPC, also known as System Architecture Evolution, SAE, core network. E-UTRAN/LTE is a variant of a 3GPP radio-access technology wherein the radio base station nodes are directly connected to the EPC core network rather than to RNCs. In general, in E-UTRAN/LTE the functions of a RNC are distributed between the radio base station nodes, e.g. eNodeBs in LTE, and the core network. As such, the Radio-Access Network, RAN, of an EPS has an essentially “flat” architecture comprising radio base station nodes without reporting to RNCs.
ARQ—Transmissions and Feedback
One approach to handling transmission errors in a wireless communications network is Automatic Repeat reQuest, ARQ. A wireless device using ARQ will detect if a received data packet is in error or not. If not, the wireless device declares the received data error-free and notifies the network node by sending a positive acknowledgement, ACK. If an error was detected, the wireless device may discard the received data and notify the network node by sending a negative acknowledgement, NACK or NAK. In response to a NAK, the network node may retransmit the same information to the wireless device.
Today, in many wireless communication networks, a combination of forward error-correcting coding and ARQ is used. This is commonly referred to as Hybrid ARQ. Hereinafter, when referring to the term ARQ also HARQ is to be considered referred to.
The received data from a given downlink, DL, transmission and any potential retransmissions of the same data may be said to form or constitute an ARQ process. Each reception of a (re)transmission of this data generates an ACK/NACK message that is also said to belong to this ARQ process. It is important that a given ACK/NACK is associated with the correct ARQ process at the transmitting side as well, such that the correct data is retransmitted, i.e. in case of NACK, or new data may be associated with this ARQ process, i.e. in case of ACK.
For DL ARQ transmissions in LTE today, ARQ feedback, i.e. ACK/NAKs, is sent from the wireless device to the network node on either Physical Uplink Control Channel, PUCCH, or Physical Uplink Shared Channel, PUSCH, depending on whether the wireless device has been scheduled for uplink PUSCH transmission or not.
For a wireless communication network using Frequency-Division Duplex, FDD, the transmitted ARQ feedback from one downlink, DL, transmission is received by the network node in the uplink, UL, at a point in time sufficiently long after the corresponding downlink transmission to the wireless device. In the case of LTE, the timing of the transmitted ARQ feedback is such that the feedback from one DL transmission is received by the network node in the UL in subframe n+4 if the corresponding DL transmission to the wireless device was in subframe n. This corresponds to a delay of 4 ms in total. This also sets the total time budget available for the combined propagation delay in DL and UL (which may be up to 0.67 ms and is accounted for in the timing-advance procedure), together with the DL and UL processing delay in the wireless device.
For a wireless communication network using Time-Division Duplex, TDD, the delay from DL data transmission to UL feedback reception may be larger than for FDD, which in the case of LTE means larger than n+4, in order to cater for the half-duplex DL-UL split. This may also cause feedback from more than one reception-time instant or ARQ process to be transmitted at the same time. However, regardless of whether the wireless communication network uses FDD or TDD, the network node may still act in a predictable manner, i.e. the delay from DL transmission to ARQ feedback reception is fixed and determined in standard specifications.
It may here also be noted that in a wireless communication network using dynamic TDD, an asynchronous ARQ protocol with on-demand ARQ feedback may be needed. In this case, the delay from DL transmission to ARQ feedback reception is not necessarily fixed and determined by the specifications, but instead given by the timings of an ARQ request and corresponding feedback.
Downlink Decoding Delay in the UE
As indicated above, the allowed total processing delay in a wireless device is fixed and determined for a certain uplink timing advance as defined by the standard specifications. This means that the processing delay in the wireless device from the determination of the ARQ feedback information pertaining to a downlink reception and up until the determined ARQ feedback information is transmitted is determined and fixed from a network point of view. In some sense, this reflects a “worst-case” scenario with respect to the decoding delay of the downlink in the wireless device. In many cases, however, this delay could be much smaller than the 4 subframes a 1 ms which some wireless communications networks are configured for today.